Previous work (Thorgaard, G. H. et al., Aquatic Toxicology 46:121-126, 1999)
showed triploid rainbow trout (0. mykiss) given embryonic carcinogen bath exposures
had significant reduction of induced tumors relative to diploids. In the present study,
trout were made triploid by thermal shock after fertilization. At age of 5 months they
were given dietary carcinogen: aflatoxin B1 (AFB₁) or 7,12-dimethylbenz[a]anthracene
(DMBA) for 30 or 120 days. The dietary exposures were at known tumorigenic levels
(100, 200 and 300 ppb AFB₁; 250, 500 and 850 ppm DMBA). At about 16 months after
fertilization the fish were sacrificed and tumor incidence and multiplicity were assessed.
At all levels of carcinogen and in all tumorous organs tumor incidence was lower in the
triploid fish. For DMBA-fed fish it was seen that the diploid:triploid incidence ratios
ranged from 2.0 to 9.0 and for AFB₁ from 3.1 to 6.0. Weight class analyses dissociated
the tumor incidence effects of growth from the effects of triploidy. Weight classes
plotted against logit tumor incidence at all doses and durations showed parallel logistic
lines. In every case the triploid curve was substantially lower than the diploid curve,
showing the independent suppressive effect of triploidy.
Fifteen triploid DMBA liver tumors were examined by direct cycle-sequencing
of p53 PCR products across the exons 5, 7 and 8 known to contain nearly all human
tumor p53 mutations. There were no p53 mutations seen at, or above, the present
threshold of detection, (for radiolabeled manual sequencing, under 5% of mutant in
normal). Fluorescent sequencing of 15 stomach tumors, also showed no p53 mutations
in the hotspot-containing exons. Mutation detection by sequencing the trout Ki-ras1
gene, ortholog human KRAS2, showed codons 12, and 61 mutations in DMBA-fed trout
liver and stomach tumors. The DMBA liver tumor Ki-ras1 mutation incidence showed
no change by ploidy. There was a significant reduction in Ki-ras1 exon 1 mutations in
triploid stomach tumors (5% in triploids v. 33% in diploids, Fisher's Exact test p<O.O5).
AFB₁ liver tumors showed Ki-ras1 mutation incidence of 75% (9/12) in diploids and
90% (9/10) triploids, nearly all in exon 1, this mutation difference with respect to
ploidy did not reach significance. / Graduation date: 2001
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/10581 |
| Date | 06 November 2000 |
| Creators | Ford, Bryan L. |
| Contributors | Bailey, George S. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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